This invention relates to the cooling of gas turbine components and, more specifically, to the cooling of platform areas of gas turbine buckets.
Turbine buckets include an airfoil region and a hollow base or shank portion radially between the airfoil and an assembly end such as a dovetail by which the bucket is secured to a turbine rotor wheel. A relatively flat platform lies at the base of the airfoil and forms the top surface or wall of the hollow shank portion.
The airfoil has leading and trailing edges, and pressure and suction sides. The airfoil is exposed to the hot combustion gases, and internal cooling circuits within the airfoil itself are commonly employed, but are not part of this invention. Here, it is cooling of the bucket platform that is of concern.
Low Cycle Fatigue (LCF) is one of the failure mechanisms common to all gas turbine high-pressure buckets. Low cycle fatigue is a function of both stress and temperature. The stress may arise from the mechanical loading, or it may be thermally induced. Diminishing the thermal gradients in order to increase LCF life of the component, by incorporating optimal cooling schemes, is a challenge encountered by gas turbine component designers.
While the platform area on the external gas path side of the bucket is being exposed to hot gas temperatures, the bottom of the platform is subjected to relatively low temperatures due to the air leaking from the forward rotor wheel space through a radial pin. This temperature difference between the bottom and top of the platform leads to a large thermal gradient and high stress field and therefore requires an optimal cooling scheme to reduce the thermal stresses in the platform area.
This invention relates to a unique methodology in designing the required bucket platform cooling hardware, including an impingement plate located within the hollow bucket shank, beneath the bucket platform. The impingement plate is spaced a substantially uniform distance from the surface (i.e., the target surface), and includes an optimized array of impingement cooling holes divided by a rib to thereby establish impingement zones on the pressure side of the bucket platform.
The cooling methodology consists of air being fed by wheelspace flow which is pumped up toward and through the plate, with the post-impingement flow being discharged via optimally located rows of film holes drilled through the platform wall, also on the pressure side of the bucket.
The invention includes systematically defining the most efficient combination of hole diameters, hole spacing and the optimal separation distance of the impingement plate from the cooled platform under-surface. The rib bifurcating the impingement zones is designed to diminish the impact of two-dimensional cross-flow degradation on the local heat transfer coefficients. Subdividing the target surface into three different impingement zones also aids in the following:
(a) Controlling the static pressure variation in the post-impingement region.
(b) Controlling the momentum flux between the jet flow and cross-stream flow; and
(c) Optimizing the required magnitude of the heat transfer coefficients based on the varying thermal stress distribution of the target surface.
In addition to the cooling configuration and optimized jet array in the impingement plate, the platform wall itself is optimized for a varying wall thickness configuration. In order to balance the stress distribution on the pressure side of the platform and airfoil-platform fillet area, the platform thickness is varied along the axial direction. A lower uniform thickness on the leading edge side of the platform, and a higher uniform thickness on the trailing edge of the platform has been proved to be the best configuration, based on experimental studies. The platform thickness along the tangential direction may or may not be varied.
Accordingly, in one aspect, the invention relates to a turbine bucket comprising an airfoil extending from a platform, having high and low pressure sides; a wheel mounting portion; a hollow shank portion located radially between the platform and the wheel mounting portion, the platform having an under surface; and an impingement cooling plate located in the hollow shank portion, spaced from the under surface, the impingement plate having a plurality of impingement cooling holes therein.
In another aspect, the invention relates to a gas turbine bucket comprising an airfoil extending from a platform, having high and low pressure sides; a wheel mounting portion; a hollow shank portion located radially between the platform and the wheel mounting portion, the platform having an under surface; means for enabling impingement cooling of the under surface, and means for discharging cooling air from the hollow shank portion.
In still another aspect, the invention relates to a method of cooling a turbine bucket platform located radially between an airfoil and a mounting portion, the platform forming a radially outer wall of a hollow shank portion comprising fixing an impingement cooling plate within the hollow shank portion, spaced from an under surface of the platform, the impingement cooling plate having a plurality of impingement cooling holes therein; providing discharge holes in the platform; and directing turbine wheelspace air flow through the impingement cooling holes and the discharge holes in the platform.